1. Field of the Invention
The present invention relates to zero-voltage-switched, isolated converters, and more particularly to an improved zero-voltage-switched, transformer coupled converter that utilizes the inductance of the transformer as a resonant element to provide zero-voltage-switching.
2. Description of the Prior Art
Circuit parasitics, such as transformer leakage inductance, semiconductor junction capacitances, and rectifier reverse recovery are among the major factors impeding higher switching frequency of "hard switched" pulse-width-modulating (PWM) converters. Recent developments in high-frequency resonant and quasi-resonant converter topologies have shown an increased utilization of parasitic components. Several innovative techniques have been proposed to operate the active switch with zero-voltage turn-on in order to minimize switching losses, stresses, and noise. The zero voltage-switching concept has also been extended to include both active and passive switches. A generalized multi-resonant concept was proposed, enabling implementation of zero-voltage-switching (ZVS) for all semiconductor components. The resulting multi-resonant converters (MRCs) utilize the transformer leakage inductances and transistor and rectifier junction capacitances to form a multi-component resonant network in order to obtain zero-voltage turn-on of all semiconductor components, reducing switching losses.
Zero-voltage-switched quasi-resonant converters (QRCs) and multi-resonant converters (MRCs) require the use of a relatively large resonant inductor to achieve zero-voltage switching over a wide-load range. The resonant inductor is also subjected to high resonant currents and voltages. As a result, the resonant inductor has a significant impact on the overall efficiency and size of the zero-voltage-switching, quasi-resonant-converters and zero-voltage-switching, multi-resonant converters.
In isolated zero-voltage-switching and quasi-resonant-converters and zero-voltage-switching, multi-resonant converter topologies, the resonant inductor is connected in series with the primary winding of the power transformer as shown in FIG. 1 for a half-bridge (HB) zero-voltage-switching, quasi-resonant-converter. However, the magnetizing inductance cannot be used as a resonant element, because the transformer is shorted during the resonant stage.